This invention relates to optical fibers, and, more particularly, to the winding of an optical fiber pack upon a bobbin so as to improve the thermal stability of the optical fiber pack.
Optical fibers are strands of glass fiber processed so that light transmitted therethrough is subject to total internal reflection. A large fraction of the incident intensity of light directed into the optical fiber is received at the other end of the optical fiber, even though the optical fiber may be hundreds or thousands of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the optical fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide.
Glass optical fibers are typically fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage. As an example of the dimensions, in a typical configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the optical fiber plus the polymer buffer is about 250 micrometers (approximately 0.010 inches).
For such very fine optical fibers, the handling of the optical fiber to avoid damage that might reduce its mechanical strength properties becomes an important consideration. In one approach, the optical fibers are wound onto a cylindrical or tapered cylindrical bobbin, generically termed a frustoconical bobbin, with many turns adjacent to each other in a side by side fashion. After one layer is complete, another layer of optical fiber is laid on top of the first layer, and so on. A weak adhesive is typically applied to the layers of optical fiber, to hold them in place. The final assembly of the bobbin and the wound layers of optical fiber is termed a canister, and the mass of wound optical fiber is termed the fiber pack. When the optical fiber is later to be used, the fiber is paid out from the canister in a direction parallel to the axis of the bobbin.
The canisters must sometimes be stored for extensive periods of time at extreme conditions. In one important application, the canisters are used in optical fiber-guided missiles. The missiles must be storable at low or high temperatures, and then immediately operable upon demand in the very low temperatures found at high elevations or in the high temperatures found in desert conditions. Procurement specifications for missiles reflect this requirement for storability and operability through temperature extremes. In one example, a missile must be storable and operable at temperatures ranging from -40.degree. C. to +55.degree. C.
When the optical fiber canister is heated and cooled at various times in testing, storage, or service, the optical fiber pack may degrade due to the thermal expansion difference between the optical fiber pack and the bobbin upon which it is wound. The optical fiber pack expands by different amounts in the axial and circumferential directions. In the axial direction, the thermal expansion coefficient of the optical fiber pack is dominated by that of the buffer material, and is relatively large. In the circumferential direction, the thermal expansion coefficient of the optical fiber pack is dominated by that of the glass, and is relatively small. Thus, there is typically a difference in either or both directions between that of the optical fiber pack and the bobbin.
Where there is a difference in thermal expansion between the optical fiber pack and the bobbin in any direction, there may develop folds, cracks, or other types of mechanical instabilities in the optical fiber pack during storage. Then, when the optical fiber is to be dispensed, the payout may be irregular so that the optical fiber may snarl. In extreme cases the optical fiber may break as a result of the instabilities created in the optical fiber pack due to the irregularities induced by the temperature changes, rendering the entire optical fiber and missile useless. Even one such fatal irregularity in thousands of meters of optical fiber is sufficient to cause such a failure. It is therefore important to take care to ensure that mechanical instability due to differences in thermal expansion is not present.
Thus, there is a need for an optical fiber canister which is not subject to formation of mechanical instabilities as a result of temperature changes during testing, storage, and use. This need is particularly acute for compact dispensers that are used to store and dispense very long lengths, typically many kilometers, of optical fiber. The present invention fulfills this need, and further provides related advantages.